The JAK2 V617F mutation has recently been described as an essential oncogenic event associated with polycythemia vera (PV), idiopathic myelofibrosis (IMF), and essential thrombocythemia. This mutation has been detected in all myeloid lineages but has not yet been detected in lymphoid cells. This raises the question whether this molecular event occurs in a true lymphomyeloid progenitor cell. In this work, we studied the presence of the mutation in peripheral blood cells and sorted B, T, and natural killer (NK) cells from PV and IMF. We detected the JAK2 V617F mutation in B and NK cells in approximately half the patients with IMF and a minority of those with PV. Moreover, in a few cases patients with IMF had mutated peripheral T cells. The mutation (homozygous or heterozygous) could be subsequently detected in B/NK/myeloid progenitors from PV and IMF, with a much higher frequency in clones derived from IMF. Using the fetal thymus organ culture (FTOC) assay, the mutation was also detected in all T-cell fractions derived from IMF and PV CD34+ cells. These results demonstrate that myeloproliferative disorders take their origin in a true myeloid/lymphoid progenitor cell but that their phenotype is related to a downstream selective proliferative advantage of the myeloid lineages.